1. Field of the Invention
The invention relates to neutron detector arrays, and specifically relates to detector arrays having a mix of detectors with and without a coating of neutron sensitive material.
2. Discussion of Prior Art
Recently, high sensitivity neutron detectors for homeland security has become increasingly important and increasingly in demand. Many known neutron detectors utilize He-3, a neutron sensitive material known to provide a detector of high sensitivity. The He-3 is provided within a volume that includes a cathode within a detection arrangement. Recently, the availability of He-3 has been has become insufficient to satisfy the demand associated with high sensitivity neutron detectors. Other than He-3 there are only a few neutron sensitive materials that are useful for constructing a neutron detector, including certain isotopes of uranium, lithium and boron.
Focusing for the moment upon the physical construction of neutron detectors and neutron detector arrangements, a neutron detector includes an anode and a cathode. One example detector includes a wire extending on an axis for the anode and a cylindrical cathode circumscribing the anode. Often, detector arrangements are configured to have a large number of individual detection pairs (i.e., a single cathode and a single anode) for high sensitivity. Also, logically, using plural detectors permits detection over a greater area that might be possible upon using just a single detector. For example, a single detector (i.e., a single anode and a single cathode) has a practical limitation on overall size.
Focusing upon boron, the majority (e.g., approximately 80%) of available boron is B-11, which has 5 protons and 6 neutrons, and the remainder (e.g., approximately 20%) is Boron 10 (B-10), which has 5 protons and 5 neutrons. Only the B-10 isotope is useful for neutron detection. Thus, for use in a neutron detector, it is typically desirable to enrich the concentration of B-10.
As mentioned, the detection of neutrons is based on the generation of secondary radiations. With B-10 (10B) as the converter material, the reaction is described as follows when a neutron is captured:10B+n→.7Li+4α(2.792 MeV, grnd state) and 7Li+4α+0.48 MeV γ(2.310 MeV, excited state)
The energy released by the reaction is approximately 2.310 million electron volts (MeV) in 94% of all reactions (2.792 MeV in the remaining 6%), and equals the energy imparted to the two reaction products (the energy of the captured neutron is negligible by comparison). The reaction products, namely an alpha particle (α) and a lithium nucleus (7Li) are emitted isotropically from the point of neutron capture by B-10 in exactly opposite directions and, in the case of the dominant excited state, with kinetic energies of 1.47 MeV and 0.84 MeV, respectively.
Turning back to physical construction of neutron detector arrangements, within a He-3 detector arrangement, each detection pair is often relatively small since the sensitivity is relatively high. This allows good resolution (i.e., the ability to discriminate neutron trajectory determination). A new generation of neutron detectors would be most beneficial if the new generation detectors provided a similar level of resolution as existing He-3 detectors without significant change to overall dimensions of the detectors. Another way of considering this idea is that the new generation of detectors must be physically similar to existing detectors so they can be easily retrofitted and must have comparable position sensitivity as a He-3 detector.
As mentioned, the use of B-10 for neutron detection is known. However, the use of B-10 in known sensor configurations (i.e., plated onto the cathode structure of known sensors) is associated with insufficient sensitivity. Specifically, B-10 coating on the cathode structure is relatively thin and such detectors achieve only a few percent efficiency, due to the fact that the thicknesses needed for a substantial capture of neutrons exceeds the escape range of the neutron capture reaction products. In one example, the optimal thickness of a B-10 coating is based upon 0.4 mg/cm2. So in many instances, capture reaction products can not escape. Only conversions of neutrons in a very thin layer near the surface of the B-10 adjacent the counting gas are detected efficiently. Since this very thin, top layer of the B-10 coating captures only a very small percentage of the incident neutrons, efficiency of a neutron detector of such simple design is understandably low.
A new generation of neutron detectors would be most beneficial if the new generation provided a similar level of neutron sensitivity without significant change to overall dimensions of the detectors. One aspect to consider for B-10 use within such a new generation of neutron detectors is an associated detection of gamma rays, which could be misidentified as detection of neutrons. Such a new generation of neutron detectors would be most beneficial and advantageous to also provide for discrimination of gamma rays.